The invention relates to a method for operating an electric arc furnace. In addition, the invention relates to a smelting plant with an electric arc furnace operated in accordance with this method.
In an electric arc furnace, pieces of material to be melted down, generally scrap metal, are melted, if necessary together with further additives, by an electric arc which is struck between the material to be melted and at least one electrode.
It has been found that unwanted disturbances of the mains supply are produced, above all, by electric arc furnaces which are fed practically exclusively with scrap. For this purpose, therefore, steel works install compensation systems, so-called SVC systems or static reactive power compensators, in order to reduce such mains supply disturbances as harmonic waves or so-called flicker, and to adhere to internationally prescribed limiting values. However, such SVC-systems react only with a lag to a mains disturbance which has already been produced, i.e. to the harmonic waves or flicker generated in the electric arc furnace during the melting down of scrap, and, particularly if the electric arc furnaces are operated on weak supply networks, cannot ensure that the prescribed limiting values are adhered to.
From the related art, a known way of avoiding high flicker values is to feed the scrap in as a mixture with a low so-called Kst value. In accordance with IEC standard 61000-3-7, this Kst value specifies the nature, weight and density of the scrap, and as a rule lies between 48 and 85. In addition to this, so-called supplementary reactances are incorporated in the furnace circuit, for example a furnace transformer restrictor, and control engineering parameterization is effected for the purpose of stabilizing the electric arc. Newer SVC systems, based on IGBT technology, are also utilized if a large improvement in the flicker values is required. Using SVC systems of this type, the frequency spectrum of the disturbance which is produced is transformed to a higher frequency range, so that it no longer falls within the measurement region laid down by the international norms for flicker disturbance to the mains supply. However, these measures involve either high investment costs or on the other hand production losses. Over and above this, it is not only the composition of the scrap and its quality which affects the disturbances of the mains supply. Rather, it is the case that the manner of operation during the melting down of the scrap also has such an influence. Thus, for example, movements of the scrap and collapsing of the scrap pile can produce disturbances of the mains supply. However, the detection of scrap movements and collapses of the scrap is largely the responsibility of the operating staff, so that regulation of the electrodes for these events is only retrospectively possible, i.e. as a reaction after a movement of the scrap has occurred and disturbances of the mains supply have already arisen.